The present invention relates to a charger for simultaneously charging a plurality of rechargeable batteries.
Many portable electronic products use rechargeable batteries as their energy source. Examples of such batteries are of the AA or AAA size, for example of the Nickel Metal Hydride (NiMH) type. In charging a rechargeable battery, electrical energy, that is an electrical current is passed through the battery in a reverse direction to the discharge current direction and is converted into chemical energy internally of the battery, which process is accompanied by generation of heat energy causing a rise in the temperature of the battery being charged, both internally and on its surface. As the battery becomes more fully charged, the internal chemical reaction slows and more of the electrical energy input is converted into heat energy. This continues until, at full charge, the chemical reaction effectively ceases and virtually all of the electrical energy input is converted into heat energy, which stage is manifested by a significant increase in the surface temperature of the battery. This heat generation during charging limits the rate at which a battery can be charged and thus the time taken to fully charge a battery. An International Electrotechnical Commission (IEC) xe2x80x9cstandard charging currentxe2x80x9d for rechargeable batteries specifies a charging current of 0.1 It (where It is the reference test current specified in IEC International Standard 61436) that will take a long time, for example 16 hours, to fully charge a rechargeable battery and thus avoids problems associated with heat generation during the charging process. Consumers in today""s world, however, are seeking to have their electronic products and thus the batteries that energise them available for the maximum amount of time possible and are thus requiring shorter and shorter charging times for their rechargeable batteries. Shorter charging times are achievable by increasing the charging current. However the greater the increase in the charging current, the greater is the heat generation problem, which can be so great as to destroy a battery.
In a current charger for AA and AAA sized batteries, a minimum charging time of approximately one hour is possible. Generally, charging times that are shorter than this carry a high risk of causing damage to or destroying the battery concerned because of the heat generated by the increased charging current necessitated by the shorter charging time. This problem is worsened in a charger for simultaneously charging a plurality of batteries, for example four batteries, because there is greater heat generation from the plurality of batteries than from one battery due to a localisation of the batteries to meet size and cost limitations for the charger for consumer acceptance.
An object of the present invention is to provide a charger of consumer acceptable size and cost for simultaneously charging a plurality, for example four, rechargeable batteries that allows a significantly increased charging rate and thereby much shorter charging time than is generally currently achievable for chargers of comparable size and cost.
The present invention provides a charger for simultaneously charging a plurality of rechargeable batteries, each battery having a nominal charging current as determined by its capacity rating in milliamp hours at one hour, the charger comprising
a base,
a charging circuit contained within the base and connectable to a power supply,
the base including means for individually mounting thereon a plurality of re-chargeable batteries, the mounting means for each battery including contacts connected to the charging circuit for contacting the terminals of the battery for passage of a charging current through the battery,
wherein the mounting means for each battery are located relative to the base and to each other such that a plurality of batteries mounted on the base are substantially exposed and spaced apart whereby the plurality of batteries are chargeable without over-heating with an average charging current of up to about twice their nominal charging current.
It has been discovered that within consumer acceptance constraints of size and cost for a charger, a plurality of rechargeable batteries can simultaneously tolerate a charging current of about twice that which is commonly considered to be maximally appropriate for a fast charge via an equivalently sized charger, provided the plurality of batteries can be adequately cooled. It has furthermore been discovered that such adequate cooling can be achieved without significantly changing the physical size of a charger compared to a said equivalently sized charger or unacceptably increasing the cost as by the addition of a cooling fan accessory. This cooling is achievable when the batteries are individually substantially exposed to ambient cooling air and are substantially spaced apart, that is, when they are maximally spaced apart according to the dictates of the shape of the base. For example, for a base that is of rectangular parallelepiped shape of substantially 120xc3x9780xc3x9734 mm, four batteries for simultaneously charging can each be aligned along a top edge of the base instead of lying parallel to each other across a top surface as in prior art chargers. Recesses, each opening to a top and adjacent side surface of the base, can be provided at each top edge for accommodating the batteries and ensuring exposure of each battery at the top and adjacent side surfaces for cooling via ambient air flow. Additionally each recess may be provided with a slot extending from a bottom surface of the slot and opening to a bottom surface of the base, thereby further exposing a battery in the recess to cooling ambient air flow. For a base of generally cylindrical or frustoconical shape for example, four batteries for simultaneous charging can be held in an upright orientation and spaced equally around the outer curved surface of the base. This arrangement also facilitates the use of mounting means for the batteries, for example brackets or clips, for holding the batteries off the curved outer surface, that is for spacing the batteries from the curved outer surface by a small amount for further exposure of the batteries for cooling.
Preferably the batteries are chargeable with an average charging current greater than 1.0 It and up to about 2.0 It, where It is the reference test current defined in paragraph 2 of IEC Standard 61434:1996, that is:             I      t        ⁢    A    =                    C        n            ⁢      A      ⁢              xe2x80x83            ⁢      h              1      ⁢              xe2x80x83            ⁢      h      
where
It is the reference test current, in amperes (A);
Cn is the rated capacity of the cell or battery as declared by the manufacturer, in ampere-hours (Ah);
n is the time base (hours) for which the rated capacity is declared.
According to the invention, the charging current may vary in a predetermined manner during the charging process. For example, the charger may charge at 4.0 It for 10 minutes and then at 1.0 It for 20 minutes.
For a better understanding of the invention and to show how it may be carried into effect, preferred embodiments thereof will now be described by way of non-limiting example only with reference to the accompanying drawings.